Method for pressing boards composed of particles



Oct. 18, 1960 F. FAHRNl 2,956,307

METHOD FOR PRESSING BOARDS COMPOSED OF PARTICLES Filed March 4, 1957INVENTOR. FRED FAHRNI ATTORNE Patented Oct. 18, 1960 METHOD FOR PRESSINGBOARDS COMPOSED OF PARTICLES Fred Fahrni, Freudenbergstrasse 1'15,Zurich, Switzerland Filed Mar. 4, 1957, Ser. No. 643,642

Claims priority, application Switzerland Mar; 26, 1956 3 Claims. (Cl.18-475) The object of the present invention is a method for pressingboards composed of particles, such as wood shavings, wood fibres orsimilar particles, provided with a bonding material, by means of aheated press provided with distance pieces between the plates of thepress. The new method is particularly suitable for working with amulti-layer press.

In the methods which have been usual up to the present, a pressure hasbeen applied to the plates of the press which is such that the press iscompletely closed in a short time. It has been noticed that boardsmanufactured by this method very often are of uneven thickness, thethickness of the thinner parts being less than the standard thicknessdetermined by the effective clearance given by the distance pieces,mostly constituted by distance strips. It was also observed that incertain cases the thickness of the board, measured over the wholesurface of the board was too small. It has now been possible todetermine the cause of these curious irregularities. After the press hasbeen closed, the increasing desiccation of the pressed parts causes thesame to shrink and it was thus naturally possible that owing to strewingirregularities, which it is practically impossible to avoid entirely,the degree of shrinkage was not the same in every part or that thisshrinkage resulted in an underdimensioning of the whole board.

The purpose of the invention is to prevent these differences inthickness of the finished boards, caused by the shrinking of the mass ofparticles placed in the press. According to the invention this result isachieved by controlling the pressing pressure in such a manner thatduring the last part of the closing stroke of the press, the closingmotion follows at least approximately the shrinkage of the part to bepressed (or of the parts to be pressed in the case of multi-layerpresses) placed in the press and resulting from the desiccation of thesame.

The idea on which the invention is based is thus that one does not closethe press completely at first, but waits until the shrinking process ofthe mass of particles to be pressed has begun. After this the press iscompletely closed in accordance with the course of this shrinkingprocess. The adjustment of the closing motion of the press according tothe shrinkage of the mass of particles during the last part of theclosing stroke results in an even subsidence of the part to be pressedover the whole surface and the aforementioned differences in thicknessof the finished board cannot occur, even when the mass of particles tobe pressed is not entirely free of strewing faults.

The pressing pressure is preferably regulated in such a manner that atthe last 5 minutes after the beginning of the pressing operation thepress is closed to within a residual distance of at the most 2.5% of theheight of the distance pieces, and that the complete closing of thepress is effected in an even manner and occupies a further time intervalof at least 3 minutes. If it is desired for instance to manufactureboards having a final thickness of 20 mm., this would mean that a presshaving ten layers would first be closed up to a residual distancetotalling 0.5 mm., which is evenly distributed over the ten layers. Inthe course of a further time interval, which is not less than 3 minutes,the press is gradually completely closed until the plates of the pressare prevented from moving further by the distance pieces.

It is already known to reduce the pressing pressure after a firstpressing period. The variation of the pressure was however always chosenin such a manner that the press was already definitively closed duringthe shrinking process of the mass of particles. It was never intended toadjust the closing motion to the course of the shrinking process. Inmost cases the pressing pressure was reduced in order to prevent thelatter from acting unnecessarily when the plates of the press werealready resting on the distance strips.

The press for carrying out the new method is provided with means whichare actuated when there is a relative motion between the plates of thepress, and which during the last part of the closing motion of the pressascertain the closing distance remaining until the press closes, inorder that the indication may be used to con trol the pressing prwsure.These means may, as shown in the embodiment described below, beconstituted by an indicating device which can be read off directly orwhich cooperates with an automatic control device for regulating thepressing pressure. In the latter case it will be advantageous to use,for detecting the residual distance in the incompletely closed press, anelectric, hydraulic or pneumatic feeler member, such as those used inother apparatus in which a given magnitude has to be controlled infunction of a distance. The control device can be designed in numerousways. It translates the element of information emitted by the distancefeeler and influences in a determined manner the means for generatingthe pressing pressure, for instance by a corresponding opening orclosing of passages for the fluid acting on the pressing piston of thepress.

An embodiment of the means actuated when there is a relative motion ofthe plates, and which, as indicated, emit an element of information forthe control of the pressing pressure, are illustrated in the drawing.

Fig. 1 illustrates a partial view of a multi-layer press with anindicating device on which the state of closure of the press can be readoif.

Fig. 2 shows a corresponding view for another posi tion of the piston ofthe press.

The part of the frame of the press visible in the draw ing is indicatedby the cipher 1. The press in question is a press with ten layers, ofwhich three are indicated. The press contains accordingly a total ofeleven heating plates 4, which are provided in the usual way with hotwater coils and rest on a press table 3. The press table is carried byseveral press pistons 2 which can be influenced hydraulically. Distancestrips 6 limit the motion of the press in the usual way.

The press is provided in addition with an indicating device comprising agraduation 8, an indicator 10 mounted on a pivot 9, an indicator leverarm 11 and a spring 12. The end of the said lever arm is kept constantlyin contact with a stop 7 secured to the press table 3 by the traction ofthe spring 12. In the fully closed position of the press (Fig. 2) theindicator 10 points to the zero of the graduation.

The method is carried out, for instance in such a manner that after theparts to be pressed 5a have been slipped in a loose or lightly pressedstate, the press is closed slowly in the usual manner. As soon as theheating plates 4 have been brought sufi'iciently close to one anotherthat there remains between the distance strips 6 and the upper heatingplates 4, i.e., per layer, a residual distance of at most 2.5% of theheight of the distance strips (i.e., of 0.5 in the case of boards havinga final thickness of 20 mm), .the pressing pressure is regulated in sucha manner that the complete closing of the press is effected graduallyand-within a time interval of for instance 5 minutes. The indicatingdevice keeps the operator exactly informed as to the degree of closureof the press. After the pointer has attained the zero point of thegraduation, the pressing pressure can be reduced further, in order thatthere may be no unnecessary pressure exercised on the distance strips.This can also be efiected automatically, with the help of a sensitiveend switch combined with the indicating device. It is obvious thataccording to the density of the parts to be pressed, the temperature ofthe heating plates and the humidity contained in the particles, thepressure variation will be given such a shape that the closing motion inthe last .part of the closing stroke of the press corresponds as closelyas possible .to the shrinking process in the mass of particles. This isachieved when the press is completely closed at the moment when theexcess humidity of the particles has evaporated and the shrinkingprocess of the same has come to an end. The completely compressed partsto be pressed 5b can be seen in Fig. 2.

The sensitivity of the indicating device can be increased at will by theinterposition of further lever arms or of multiplacating gears. In orderthat the operator may tire less quickly, it is recommended to make use,in combination with the graduation, of electric light signals ofdifferent colors which indicate when the residual distance between theplates has attained a predetermined value and when the press iscompletely closed. Easily visible numerical tables arranged in theimmediate neighborhood of thelindicting device and which give forinstance the values of the closing distances in function-of the timeelapsed, make it easier for the operator to keep to a given course ofpressure variation for a given type of board. In the case of a manualcontrol, it is suflicient to observe the indicating device andeventually a timing device. A control of the course of the pressurevariation may often beeschewed.

What I claim is: l. A method for pressing boards composed of particles,such as wood shavings, wood fibres or similar particles provided with abonding material, by means of a multilayer heated press provided withspacing pieces between the plates of said mnlti-layer press, by drying,shrinking and bonding said particles, said method comprising in sequenceapplying a pressing pressure until the thickness of the board's has beenreduced to an amount not exceeding 102.5% of the final thickness,subsequently reducing the pressing pressure and then controlling theresidual pressing pressure in such a manner that the moment when theexcess humidity of the boards has evaporated and the shrinking processof the same comes to an end coincides at least approximately with themoment of the complete closing of the press.

2. A method for pressing boards composed of particles, such as woodshavings, wood fibers or similar particles, provided with a bondingmaterial, by means of a multilayer heated press provided with spacingpieces between the plates of said multi-layer press, by drying,shrinking and bonding said particles, comprising the steps of reducingthe pressing pressure shortly before the thickness of the boards in thepress reaches the final thickness, and controlling the pressing pressureduring the drying and shrinking period of the boards, in such a mannerthat the moment of the end of the shrinkage coincides at leastapproximately with the moment of the complete closing of the presswhereby to obviate differences in the thickness of the finished boards.

3. The method according to claim 2, comprising applying a pressingpressure by closing the press to within a residual distance ofapproximately 2.5% of the height of the spacing pieces at the last fiveminutes after the beginning of the pressing operation, and completingthe closing of the press in an even manner during a further timeinterval of at least three minutes.

References Cited in the file of this patent UNITED STATES PATENTS2,037,522 Lundback Apr. 14, 1936 2,215,244 Linzell Sept. 17, 19402,719,328 Patton et a1. Oct. 4, 1955 2,761,175 Meyerson Sept. 4, 1956FOREIGN PATENTS 429,486 Germany May 29, 1926

